The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Referring now to FIGS. 1A-1C, cellular communication systems (hereinafter cellular systems) typically comprise multiple base stations and mobile stations. In FIG. 1A, an exemplary cellular system 10 comprising base stations BS1, BS2, and BS3 (collectively BS) and a mobile station (MS) is shown. In FIG. 1B, each BS may comprise a processor 30, a medium access controller (MAC) 32, a physical layer (PHY) module 34, and an antenna 36. In FIG. 1C, the MS may comprise a processor 40, a MAC 42, a PHY module 44, and an antenna 46. The PHY modules 34 and 44 may comprise radio frequency (RF) transceivers (not shown) that transmit and receive RF signals via the antennas 36 and 46, respectively.
The BSs and the MS may transmit and receive signals while the MS moves relative to the BSs. Consequently, the MS may receive signals transmitted by one or more BSs depending on the location of the MS relative to the BSs. For example, the MS may receive signals transmitted by BS1, BS2, and BS3 when the MS is located as shown in FIG. 1A.
Typically, the BSs use the same channel, frequency, and time slot to transmit signals. The MS may associate with a BS called a serving BS and receive signals transmitted by the serving BS. A channel of signals received by the MS from the serving BS is called a direct channel. Additionally, the MS may receive signals transmitted by one or more neighboring BSs. A channel of signals received by the MS from a neighboring BS is called an interference channel.
When the MS is associated with the serving BS, the signals received by the MS from the neighboring BSs may interfere with the signals received by the MS from the serving BS. The resulting interference is called inter-cell co-channel interference (hereinafter interference). The quality of communication between the MS and the serving BS may degrade more due to the interference than due to noise when cell size decreases.
To reduce the interference, most cellular systems use different frequencies, time slots, or codes to transmit signals from different BSs so that the signals transmitted by multiple BSs are almost orthogonal to each other. Since cross-correlation between signals that are almost orthogonal is nearly zero, the interference between the signals is reduced.
For example, systems utilizing Orthogonal Frequency Division Multiplexing Access (OFDMA) use pilot tones that are almost orthogonal, and systems utilizing Code Division Multiple Access (CDMA) use codes that are almost orthogonal. In the OFDMA systems, the pilot tones are generated using pseudo-random noise (PN) sequences. The PN sequences, however, may have some non-zero cross-correlation. Consequently, the pilot tones may not be perfectly orthogonal, and some interference may persist. Similarly, some interference may persist in CDMA systems when the codes are not perfectly orthogonal.